I like to AM DX during the summer (mostly baseball games). The Grundig external jacks require a 500 ohm antenna. Would my would my 20-40-80M trap dipole that I use with my ham rig, match up somehow to the Grundig?

I do not think you will notice one bit of difference on receive if you use a 500 ohm antenna or a 50 ohm antenna or a 5000 ohm antenna. For AM broadcast, I think a long wire as long and as high as possible will be your best bet.

I have a Grundig 880 (or is it 800). Any way it works great on AM BC with a long wire.

To be honest, I only have this SWL radio. It is my first and only one. I have typically just used the internal, and so much as never looked at the back. I put up a trap dipole last month. So when I when to look at the back of the radio, I saw 500 ohm. I have never seen that type of antenna before.

Perhaps that's a mistake and the radio should have said 50 ohms. Agree with AD4U. Even if there's a severe mismatch with losses of 10 or even 20 dB that won't reduce your ability to hear AM broadcasts. Turn up the volume to offset that.

Consider too that the portable may work better with only the internal ferrite rod antenna which gives you excellent ability to null QRN/QRM.

It probably has two antenna terminals designed for a balanced feed line (300 - 600 Ohm, aka 500 Ohm). If you connect coax then you need to put the shield on one terminal and the center conductor on the other. Sometimes there is a third chassis ground terminal that can be strapped to one side of the balanced input so that you can connect a single random wire antenna (unbalanced) to the other terminal. At any rate, the actual antenna impedance is not very critical.

Severe mismatches might reduce the OP's ability to hear weak AM stations or the DX he wants!Also, overloading can cause desensitizing of the radio and static discharges can damage it. These are common problems with portable radios and long wire antennas. Use of a loop antenna is usually a good compromise.

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